Problem: $ { {5} \times \left[\begin{array}{rr}{2} & {1} \\ {3} & {-1}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{5}\times{2} & {5}\times{1} \\ {5}\times{3} & {5}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{10} & {5} \\ {15} & {-5}\end{array}\right]}$